When he was young, the professional boxer worked as a prize-fighter competing for cash rewards.

prize-fighter 是由 prize（奖金、奖品）和 fighter（战士、拳击手）组成的复合名词。prize 指比赛或竞赛中获得的奖励，fighter 指参与战斗或比赛的人。 prize-fighter 特指"为奖金而战的职业拳击手"，在历史上指那些参加公开拳击比赛以赢取奖金的职业选手。